HelpMePlease123

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The files go in the very latest patchday that has a vehicles.rpf.  The reason some car files still say "patchday 3" is because in the past, previous patchdays worked.  They don't anymore.  So you should be fine as long as you don't have a patchday above 14.
 
When you double click on the .yfts that you've replaced the vehicles of, does it show the replaced car, or the GTA V vanilla?  If it shows the replaced, this means that somewhere the vanilla files are still overriding the mods folder, which shouldn't be the case.
 
My first suggestion is to roam through the previous patchdays in your mods folder and remove other files of the same name.  This way, you will only have one set of .ytd, .yft, and_hi.yft for that car slot.

Yup, that is the best way to set up a crime/accident scene that you want to drive to for example.
 
 
I'll post a slightly detailed step by step guide.  Relies heavily on Simple Native Trainer
 
(Start)  Spawn Car.   Then set how dirty the car is(some people are picky about this) =D
 
Set how you want ELS to look.  -  (Optional) Freeze Car (I prefer this as I don't want to drive across the city and find out that the stupid AI drivers pushed the fire engine into the ocean...)
 
Save the Car,  and Delete Car Blips as the last step.  (when you save the car,  a floating marker/chevron appears and stays above the vehicle.  You'd probably want to remove it if you are making a video for example)